The present invention relates generally to optical devices such as polarizers, diffusers, and mirrors, and more particularly to improvements in the materials used to make such devices.
Various optical films and devices are known to the art which rely upon refractive index differentials, sometimes produced by strain-induced birefringence, to achieve certain optical effects, such as the polarization of randomly polarized light. Such films and devices may be in the form of a multilayer stack in which index differentials between adjacent layers in the stack give rise to certain optical properties, as in the films disclosed in U.S. Pat. No. 5,882,774 (Jonza et al.). Other optical devices comprise a disperse phase which is disposed in a continuous matrix, and derive their optical properties from refractive index differentials between the continuous and disperse phases. The materials disclosed in U.S. Pat. No. 5,825,543 (Ouderkirk et al.) are representative of this type of a system. Various hybrids of the aforementioned systems are also known, such as the multilayer optical films disclosed in U.S. Pat. No. 5,867,316 (Carlson et al.), wherein the film comprises a multilayer stack having a repeating layer sequence in which at least one of the layers has a continuous phase/disperse phase morphology. Various other optical films and devices are also known to the art, and are described in U.S. Pat. No. 5,831,375 (Benson, Jr.), U.S. Pat. No. 5,825,542 (Cobb, Jr. et al.), U.S. Pat. No. 5,808,794 (Weber et al.), U.S. Pat. No. 5,783,120 (Ouderkirk et al.), U.S. Pat. No. 5,751,388 (Larson), U.S. Pat. No. 5,940,211 (Hikmet et al.), U.S. Pat. No. 3,213,753 (Rogers), U.S. Pat. No. 2,604,817 (Schupp, Jr.), Aphonin, O. A., xe2x80x9cOptical Properties of Stretched Polymer Dispersed Liquid Crystal Films: Angle-Dependent Polarized Light Scatteringxe2x80x9d, Liquid Crystals, Vol. 19, No. 4, pp. 469-480 (1995), Land, E. H., xe2x80x9cSome Aspects of the Development of Sheet Polarizers, (copyright) 1951 Optical Society of America, Reprinted from Journal of the Optical Society of America, Vol. 41(12), 957-963, (December 1951), pp. 45-51 and 2244 Research Disclosure (1993), July, No. 351, Emsworth, GB, xe2x80x9cPolarizerxe2x80x9d, pp. 452-453.
In the past several years, a number of advances have been made in the materials sciences, especially in the area of block copolymers, which have resulted in the development of new and interesting materials and methods for making and using these materials to various ends. In some cases, these advances have led to applications in the field of optical films and devices. Thus, for example, Urbas et al., xe2x80x9cOne-Dimensional Peroidic Reflectors from Self-Assembly Block Copolymer-Homopolymer Blends,xe2x80x9d Macromolecules, Vol. 32, pages 4748-50 (1999), report the formation of well ordered photonic crystals similar to a multilayer quarter wave stack comprising self assembling blends of block copolymers optionally containing homopolymers. One embodiment describes the formation of a narrow band reflector. Also summarized is the use of neat block copolymers as well as copolymers comprising liquid crystalline materials as means of producing periodicities in block copolymer materials.
U.S. Ser. No. 08/904,325 (Weber et al.)(corresponding to WO 9906203) discloses the transesterification or reaction of polyesters lying in adjacent layers of a multilayer optical stack for the express purpose of improving interlayer adhesion. It is assumed that the thickness of the interface comprising the reacted materials is sufficiently thin so as not to otherwise affect the optical properties of the optical stack except at the interface.
U.S. Ser. No. 09/006,455 (Merrill et al.)(corresponding to WO 9936812) discloses the use of transesterified blends of PEN and PET within a single layer in a multilayer optical stack for the purpose of producing optical devices such as polarizers and mirrors.
U.S. Pat. No. 3,546,320 (Duling et al.) discloses transesterification methods for preparing a semicrystalline composition comprising 94 to 60 weight percent polyalkylene terephthalates, 6 to 40 weight percent polyalkylene naphthalene-2,6-dicarboxylate, and at least 5 weight percent of a block copolymer comprising discrete polymer segments of the percent polyalkylene terephthalate and the polyalkylene naphthalene-2,6-dicarboxylate. The block copolymer is prepared by melt transesterification of the individual homopolymers, and the degree of transesterification is controlled by the mixing time. Duling demonstrates a total loss of crystallinity of the block copolymer after extensive transesterification, depending on the composition.
U.S. Pat. No. 3,937,754 (Sagamihara et al.) discloses a biaxially oriented polyethylene-2,6-naphthalate (PEN) film containing a polyester resin other than PEN in an amount of 0.5 to 10 percent by weight based on the PEN, and a process for its production. The reference notes that when the PEN resin (1) is blended in the molten state with a polyester resin (2), the softening point of the blended mixture decreases gradually from the softening point of the PEN until it finally reaches a certain point, referred to as an equilibrium softening point. The reference teaches that this softening point coincides with the softening point of a PEN copolymer obtained by copolymerising monomers of the same composition and proportion as the monomers which constitute the PEN resin (1) and the polyester resin (2). From this fact, the reference presumes that reaction occurs via a stage of forming a block copolymer, where given enough reaction time a copolymer will be obtained.
Research Disclosures 28,340 and 29,410 disclose transesterified products of PEN, PET, and other polymers comprising dibasic acids. Typical dibasic acids include isophthalic, adipic, glutaric, azelaic, and sebacic acid and the like. The PEN based polymers are generally based on 2,6-naphthalene-dicarboxylic acid but may be based on 1,4-, 1,5-, or 2,7-isomers or mixtures of these isomers. These teachings primarily address the ability to control mechanical and physical properties such as modulus, gaseous permeabilities, and glass transition temperatures.
WO 92/02584 (Cox et al.) disclose the use of phosphite materials to control the rate of transesterification during solid state polymerization, primarily for the intended use of improving physical and mechanical properties, such as gaseous diffusion, in the final product application. The reference discloses blends of PEN and PET homopolymer pellets, which are held at a temperature range between the higher glass transition temperature and the lower melting temperature.
Despite the many advances noted above in the area of optical films and devices, a number of problems still persist in the art. For example, it is often desirable to rely on strain-induced birefringence to achieve desirable optical properties in an optical film, since the film can be conveniently oriented in a controlled manner on a laboratory stretcher in accordance with well established methodologies and principles. However, these methodologies do not work equally well for all materials selections. In particular, problems are frequently encountered with the use of thermodynamically immiscible polymers whose interfacial strength is not large, because the resulting film cannot always be stretched to a high enough draw ratio to achieve an optimal level of birefringence. In the case of a continuous/disperse phase system, for example, orienting such a film to the draw ratios required for optimal birefringence may lead to voiding at the interface between the two phases, thereby compromising the desired optical properties (e.g., polarizing properties) of the system. Voiding of this type is described in U.S. Pat. No. 5,811,493 (Kent), where it is used to produce paper-like films which are diffusely reflective to both polarizations of light. Unfortunately, if lower draw ratios are used to prevent voiding, the resulting film may have a lower degree of birefringence and less than optimal optical properties.
There is thus a need in the art for a method for achieving a desired degree of birefringence in an optical film or device while reducing the draw ratio normally required to achieve the desired level of birefringence. There is also a need in the art for a method for making optical films and devices from thermodynamically immiscible polymers whose interfacial strength is not large, wherein the films and devices are capable of being oriented to the higher draw ratios frequently required to achieve a higher degree of birefringence and optimal optical properties. These and other needs are met by the present invention, as hereinafter described.